1. Field
The present invention relates to mobile devices and more particularly to a method and apparatus for accurately determining the altitude of a mobile device.
2. Background
Position determination is an important and growing feature of cellular telephones and other handheld mobile devices. The most prevalent approach to determining location is to embed Global Positioning System (GPS) navigation receiver capability into the mobile device. However, GPS-based positioning has limitations. GPS systems cannot resolve altitude accurately. In some larger and more complex devices, the GPS systems have been augmented with barometric pressure based altimeters. This provides a significant improvement in altitude accuracy. However, a barometric altimeter responds to changes in pressure as well as changes in altitude. The altimeter can be fooled when a person enters or leaves climate-controlled rooms and buildings, or experiences other changes in environmental pressure. The present invention minimizes the effects of environmental pressure changes on the determination of altitude.
The ability to determine the location of a mobile device such as a cellular telephone is desirable for emergency calls, and for a variety of commercial purposes generally called Location Based Services. GPS-based position determination is based upon determining the distance that the signal travels from the GPS satellites, so it is not very accurate in indoor environments and other locations, where satellite signals can be blocked, distorted or reflected. In addition, because the satellites are usually high in the sky, the GPS-based position is less accurate in altitude, than in the horizontal position. This is why pressure-based altimeters have been added to more complex GPS systems in the past. However, the addition of altimeters has not been implemented in low-cost consumer mobile devices because of the cost and size. There are several other limitations to using these prior art solutions.
Local atmospheric pressure can change for three reasons—change in true atmospheric pressure (weather-related barometric pressure changes), changes in the environment (going into a building where the ventilating system maintains a higher or lower pressure), and changes in altitude. Separating these other effects to determine true changes in altitude is difficult. Additionally, the sensitivity of low cost atmospheric pressure sensors varies with their temperature, so changes in temperature can be misinterpreted as changes in altitude.
A solution to these problems is needed that would enable the use of atmospheric pressure sensors in mobile devices.